blow back

[Nightengale]

what causes blow back thru the carbs. sometimes i get a blow back thru my carbs, and it is very random, also some times when i turn on the ignition i bet a blow back. was just wondering since i have had a few carb boot vent covers blow off. i have not set up the vaccume advance on my fj carbs yet (one of my winter projects, one of many). it runs pretty good without the vac advance, but i am thinking it is the cause of the blow back. by the way i have pod filters and a straight exhaust. thanks.

[WMarshy]

http://www.xs11.com/forum/showthread.php?t=37018&highlight=huff

Is it anything like described here?

[Nightengale]

Yes

Yes it does do just like that, But also it does it at low speeds sometimes too.

[crazy steve]

There could be any number of reasons for backfires through the carbs. Ignition timing, the pod filters, lack of backpressure with the straight pipes, fuel mixture, or more likely some combination of the above. The straight pipes are probably the biggest problem, as low-speed issues are common on most any motor with insufficient backpressure.

From what's been reported here over time, IMO these motors 'tuning window' gets narrower the more you get away from the stock intake/exhaust setup with jetting becoming more critical. And even then, some driveability issues can't be tuned away. Open exhaust only makes maximum power at WOT/high RPM, with reduced power at lower speeds (this has been proven on Dynos). Yamaha spent a lot of time 'tuning' the OEM intake and exhaust for the best power across the power curve, and changing anything (while sometimes increasing max power) usually results in a power loss somewhere on the curve.

The 'fix'? Because there's very few (if any) owners running your combination to give you direction, about all you can do is experiment with different jetting in the carbs and that will likely include all the jets, not just the mains. Float height can affect this, as will needle jet type/position. A lot of combinations to explore (some dyno time would shorten the process) and a sizeable investment in parts. I don't think the lack of the vacuum advance is the problem (it may make it worse), but connecting it should help with mid-range power.

[crazy steve]

There is one lo-buck trick you could try to 'tune' your open exhaust that may help. Pipe length has a pretty big effect on exhaust resonance, with too long or short pipes causing problems. A trick the open-header drag racers use is to make the pipe long, then paint a stripe of white paint on it. At the point where the paint isn't discolored/burned, cut the pipe 1" beyond that into the undamaged paint. That's supposed to be the optimum length, might be worth a try....

[Nightengale]

thanks

Thanks for the advice Steve. I do have some baffles on order for the exhaust I was hoping that it should help.

[crazy steve]

If you're going to install baffles for backpressure (a good move), still try the paint trick and make sure the baffles are in a 'good' paint area, not into the burned part.

[Nightengale]

I like that idea

I think I'm going to try that. What diameter pipe do I need to connect onto the header? I have the regular 2 into 1 exhaust

[crazy steve]

I don't remember what exhaust you have; if you have the stock head pipes, it will depend on where the mufflers were cut off because of the taper. And the pipe has to be big enough for the baffles to go in, but not so big that the baffles are 'loose'. I wouldn't go smaller than 1.5", personally I'd probably use 1.75" OD pipe. That may require an adaptor to mate with the head pipes...

[Nightengale]

All I did was remove the mufflers I didn't cut the header pipe at all.

[crazy steve]

Well, the exhaust necks down pretty small before going into the mufflers, so you'll probably need the adaptors to get to a 'standard' pipe size. I suspect some welding will be involved. I know that the 'replacement' mufflers that Jardine used to sell required that you cut the mufflers in a specific spot in order to get the needed diameter, that's also the case when using Harley mufflers.

If your pipes are cut off right behind the collector, you'll definitely need to extend them. Trying to install a baffle into the collector will give uneven backpressure on the paired cylinders because of it's internal construction, so don't try that.

[skids]

I know that the 'replacement' mufflers that Jardine used to sell required that you cut the mufflers in a specific spot in order to get the needed diameter, that's also the case when using Harley mufflers.

Steve, I have a set of the Jardines in a box. If you can obtain that info, I would appreciate it!

[crazy steve]

Here ya go..... http://www.xs11.com/xs11-info/tech-t...-mufflers.html

All kinds of nuggets on this site...

[skids]

Thanks Steve! I got mine off fleebay about two years ago and they were supposed to be new in the box. Guess what...no instructions and no crossover pipe component.

[GLoweVA]

John (Nightengale) has the Mac 4 into 2 headers. He just pulled the Mac mufflers off.
Same as I used to have (until TC made me replace headers), but I can't recall off the top of my head the pipe diameter size.